AEROBIC DIGESTERS IN AN OXIDATION DITCH CONFIGURATION: AN UNEQUIVOCAL SUCCESS

The Harpeth Valley Utilities District completed construction of an aerobic digester system in an oxidation ditch configuration system in February 2001. The system consists of two digesters operated in series with a third digester to provide digested biosolids storage. An inreactor thickener increases the solids concentration of digested sludge before it is pumped to the storage digester. Two approaches were used to calculate the required digester volume to achieve volatile solids and pathogen reduction requirements set forth in 40 CFR 503: a conversion using the target solids retention time and digester biosolids mass, and a derivation from a mass balance on solids using a first order biological reaction for the destruction of solids. The former method, which was ultimately selected for process design, resulted in a more conservative design with a larger digester volume. Thirteen months of operational data have demonstrated that the digesters are exceeding all expectations for digester solids concentration, volatile solids destruction, and pathogen removal. With the digester hydraulic retention time currently at 120 to 140 days, digester total suspended solids are approaching a four percent concentration. Digested solids are approximately 45 percent volatile, which is well below the target of 55 percent. Most notably the maximum pathogen concentration recorded to date is below 20,000 colony forming units (CFU) per milliliter, which is two orders of magnitude below the ceiling limit of 2,000,000 CFU/mL. Digested sludge is thickened by gravity to 3.5 to 4 percent solids. Operational and mechanical problems have been few, and the plant operators are very pleased with the system.